1. Field of the Present Invention
The present invention relates generally to electric field sensors, and, in particular, to signal stabilization in a non-contact resistive contact sensor assembly.
2. Background
Conventional electrodes act as a current transducers converting ionic currents into electronic ones so electrophysiological status can be assessed. The uses for this approach are many and broadly range from assessment of neural (EEG), cardiac (ECG), and skeletal (EMG) muscle activity.
This approach requires conductive contact with the source and has inherent problems. The first of these is the requirement of clean skin exposure. This requirement may compromise continuous usability due to the effects of environmental contaminants, both on the skin and in the atmosphere; extremes of temperature and their resulting general effect on skin due to physiological reactions such as “goose bumps” and excessive sweating as well as other phenomena; and potential reactions to conductive materials. The process of preparing skin and securing a good conductive contact can also decrease compliance, especially in if intended for continuous day to day use. Furthermore, during exercise, the physicality can result in electrodes being displaced. Other issues include shorting between electrodes, especially when placed in close proximity to each other, and charge transfer which has potential safety implications as well as the issue of the measurement process corrupting the signal.
The problems, outlined above, may be at least partially solved by the use of capacitive electrodes (non-resistive contact sensors) as they acquire signals through capacitive coupling, not requiring resistive contact with the source. They provide many benefits, including the fact that no electrical contact is required, and so no skin preparation or conducting pads are necessary and they can be readily moved or relocated to get an optimal signal. In addition, they can be miniaturized, they have very low power requirements, and they can be embodied as passive electric field sensors with the result that adjacent sensors do not interfere with each other.
The use of capacitive electrodes for electrophysiological monitoring is not a recent innovation, with Richardson describing it for acquisition of the cardiac signal in 19671. This system was, however, flawed being prone to problems including poor signal to noise ratio, voltage drift, electrostatic discharge and parasitic capacitance. These are still problems with capacitive sensor technologies today. Many of those problems have been addressed, at least partially, but problems with signal stability interference still plague this technology. Signal stability interference is especially problematic during movement. Movement may lead to a variety of issues that may compromise continuous signal acquisition including contact electrification between the body surface and the sensor electrode; charge build-up on the body resulting in baseline shift and potential saturation if occurs too rapidly; and movement of the sensor relative to the body that can also lead to baseline shift and saturation (railing). 1 The insulated electrode: a pasteless electrocardiographic technique. Richardson P C. Proc. Annu. Conf. on Engineering in Medicine and Biology 7: 9-15 (1967)
When dry contact electrodes are placed in direct contact with a person, and particularly when they are moved, triboelectric effects (electrical charges created by sliding friction and pressure) are frequently generated. Triboelectric effects of this nature may cause contact electrification where static charges may be delivered to the pick-up electrode. This static charge can produce a near-direct current (DC) or very low frequency drift in the sensor that may interfere with the physiological alternating current (AC) signal that is being measured or may saturate the sensor causing railing, after which the sensor takes time to return to being able to produce a useful physiologically-relevant output. If the electrode moves relative to the body, it will also pick up a geoelectric displacement signal. That is, the effect of the body, an electrically active structure, moving through the geoelectric field, which is on the order of 100 Vm−1, will cause relative polarization of the sensor that will displace the baseline and may cause the sensor to saturate. An additional source of interference is that of clothing moving on the body. As clothing moves on the body, charge separation can occur when materials that are separated on the triboelectric series donate or receive electrons from each other. After a material becomes charged it may discharge onto the surface where an electric potential is being measured, thereby interfering with signal acquisition.
Various issues can arise as a result of these various forms of interference. For example, issues may arise in the signal acquisition phase due to corruption of the signal from local electrical activity, in the signal referencing phase due to poor referencing of the signal to an appropriate earth, and during the transfer of the signal to processing units where the signal may be susceptible to interference. Thus, a need exists for devices, methods, and/or systems for reducing interference and stabilizing the signals being acquired and processed.